1. Field of the Invention
The present invention relates to a surface texture measuring instrument. For example, the invention relates to a surface texture measuring instrument that measures a surface texture such as a profile or surface roughness of a workpiece by an oscillation-type force sensor.
2. Description of Related Art
As a surface texture measuring instrument that scans a surface of a workpiece and measures a surface texture such as a profile or surface roughness of the workpiece, a roughness measuring machine, a contour measuring machine, a roundness measuring machine, a coordinate measuring machine and the like are known.
In the measuring machines, an oscillation-type force sensor is used as a sensor for detecting a workpiece surface based on fine displacement caused when a contact portion contacts the workpiece surface.
<Oscillation-type Force Sensor>
As shown in FIG. 10, an oscillation-type force sensor 1 (a measuring portion) includes a metal base 2, a stylus 3 integrally formed with the base 2, an oscillation element 4 that oscillates the stylus 3 (in an axial direction of the stylus 3) and a detection element 5 that detects an oscillation state of the stylus 3 and outputs a detection signal. A sensing pin 6 as a contact portion is adhered and fixed on a tip end of the stylus 3, the sensing pin 6 made from diamond-tip, ruby or the like. The oscillation element 4 and the detection element 5 constitute a measuring force detecting unit that detects a measuring force when the sensing pin 6 of the stylus 3 contacts the surface of the workpiece. The oscillation element 4 and the detection element 5 are each formed by a piezoelectric element. One oscillation element 4 and one detection element 5 are adhered and fixed on a front surface of the base 2 and on a rear surface of the base 2.
As shown in FIG. 11, when an oscillation signal Pi (a voltage signal) having a specific frequency and amplitude is given to the oscillation element 4 of the force sensor 1, the detection element 5 obtains a detection signal Qo (a voltage signal) having a specific frequency and amplitude.
FIG. 12 shows change in amplitude of the detection signal Qo which accompanies a contact with the workpiece. When the oscillation signal Pi having a certain oscillation at a resonance frequency of the stylus 3 is added to the oscillation element 4 in a state where the stylus 3 is not in contact with the workpiece, the stylus 3 resonates and the detection element 5 can obtain the detection signal Qo having an amplitude Ao. When the stylus 3 contacts the workpiece W, the amplitude of the detection signal Qo is attenuated from Ao to Ax.
The attenuation ratio k (Ax/Ao) and the measuring force have a relationship shown in FIG. 13.
Herein, taken as an example is a case in which the detection signal Qo generated when the stylus 3 (the force sensor 1) contacts the workpiece W is attenuated to 90% of that generated when the stylus 3 is not in contact with the workpiece W (the attenuation ratio k=0.9). From the relationship shown in FIG. 13, the measuring force in the non-contacting state is 135 [μN].
Accordingly, when the force sensor 1 is brought into contact with the workpiece W, it is possible to measure the profile and the roughness of the workpiece W with the constant measuring force by controlling a distance between the force sensor 1 and the workpiece W using the actuator or the like such that the attenuation ratio k is always constant.
<Texture Measuring System Using Force Sensor>
FIG. 14 shows an example of a texture measuring system using the force sensor 1. The texture measuring system includes a probe 10 and a controller 20 that controls the probe 10.
The probe 10 includes the force sensor 1, an actuator 11 that advances and retracts the force sensor 1 relative to the workpiece W and a detector (having a scale and a detection head) 12 that detects a displacement amount by which the force sensor 1 is displaced by the actuator 11 (measuring point information on the workpiece W when measured by the force sensor 1).
The controller 20 includes an oscillator 21 that gives the oscillation signal to the force sensor 1 in order to oscillate the force sensor 1, a peak hold circuit 22 that converts the detection signal from the force sensor 1 into a direct-current signal, an processing unit 23 that computes a deviation between an output from the peak hold circuit 22 (a force sensor signal) and a target measuring force, a force control compensator 24 that is input with an output from the processing unit 23, a drive amplifier 25 that drives the actuator 11 based on an output from the force control compensator 24 and a counter 26 that counts a signal from the detector 12 and outputs the measuring point information of the force sensor 1 as a position measurement value.
In FIG. 14, when the stylus 3 of the force sensor 1 is brought into contact with the workpiece W, the detection signal at that time is output from the force sensor 1. The detection signal is converted into a direct-current signal by the peak hold circuit 22 and then given to the processing unit 23. The processing unit 23 calculates the deviation between the detection signal from the peak hold circuit 22 (the force sensor signal) and the target measuring force. The deviation is multiplied by a gain of the force control compensator 24 and the result is given to the drive amplifier 25, so that the actuator 11 drives such that the deviation is eliminated.
<Method for Using Force Sensor as Probe>
FIG. 15 shows changes in the detection signal from the force sensor 1 (the force sensor signal) which is generated when the force sensor 1 moves from the non-contacting state to a contacting state.
When the force sensor 1 is brought into contact with the workpiece W and further pressed to the workpiece W, the detection signal from the force sensor 1 (the force sensor signal) gradually drops and becomes substantially coincident with the target measuring force to be stable in this state. In the state, when the force sensor 1 and the workpiece are relatively moved along the surface profile of the workpiece, since the detection signal from the force sensor 1 and the target measuring force are maintained to be substantially coincident with each other, the profile or the roughness of the workpiece can be scanning-measured with the constant measuring force by collecting the position measurement value from the detector 12.
<Method for Using Force Sensor as Touch Probe>
In the texture measuring system of FIG. 14, when the attenuation ratio becomes a desired value, the force sensor 1 can be used as a touch probe with the constant measuring force by incorporating a circuit that latches a current position.
As shown in FIG. 16, by comparing the force sensor signal representing the detection signal oscillation of the force sensor 1 with a contact detection level (a threshold value) for detecting a contact with the workpiece, it is possible to structure a circuit that outputs a touch signal representing a contact between the force sensor 1 and the workpiece. In this arrangement, the touch signal is generated when the force sensor signal passes the contact detection level (the threshold value), so that the same measuring force is always generated at the timing of the touch signal generation (thereby realizing higher precision).
As shown in FIG. 17, the measuring force can be also controlled by controlling the contact detection level (the threshold value) and a measurement with a lower measuring force can be enabled by raising the contact detection level, thereby realizing a ultra-precise measurement.
<Probe with Constant Force Scanning Measurement Function and Touch Measurement Function>
As understood from the above description, when the probe is used as a scanning probe or a touch probe, the force sensor can be used in a common way (under the same detection principle).
Note that there has been suggested a system that includes both functions of a scanning measurement and a touch measurement, drives a probe mounting portion by a coordinate drive mechanism that provides a three-dimensional drive and switches between the constant force scanning measurement and the touch measurement depending on the workpiece (Document: JP-A-2005-254016, for reference).
However, it is sometimes difficult to conduct a constant force scanning control due to the surface profile, the surface texture and property fluctuation owing to material of the workpiece or disturbance input to the system. When the constant force scanning control is difficult, namely when the force control is unstable, the scanning control may be oscillatory to cause variation in the measurement value, so that the high precision cannot be maintained. As a compensatory function, a switching is conducted between the constant force scanning measurement and the touch measurement.
For example, in a case shown in FIG. 18, when the scanning control becomes oscillatory during the scanning measurement, it becomes impossible to maintain the high precision due to variation in the measurement value, so that an operator judges the situation to manually switch from the scanning measurement mode to the touch measurement mode. Alternatively, when the oscillation range of the force sensor signal exceeds a preset predetermined value, a switching is conducted from the scanning measurement mode to the touch measurement mode. Further, by assuming in advance the surface profile of the workpiece, a switching from the scanning measurement mode to the touch measurement mode is automatically conducted upon an entry into a touch measurement region. Accordingly, the force sensor contacts the workpiece surface while repeating touch-back operations (i.e. operations in which the force sensor is moved away from the workpiece surface and brought into contact with the surface again) along the surface, so that the touch signal is collected at the timing of the touch signal generation.
However, in order to compensate the profile scanning measurement by the normal touch measurement, it is necessary to conduct the touch measurement at a shorter pitch than a workpiece profile cycle, which requires a longer measurement time.
Generally, the constant force scanning measurement is more advantageous than the touch measurement for measuring a fine profile, since a distance between data-measured points is smaller in the constant force scanning measurement than in the touch measurement, so that a fine profile cycle can be measured. Therefore, there have been demands for the use of the constant force scanning measurement as far as possible.